Total hip replacement is considered the most successful orthopaedic surgical procedure ever devised. Its success has now spanned three decades. There are now approximately 120,000 hip replacements performed in the U.S. each year. About one-third of the operations performed are for revision replacement of previously implanted prosthetic hips. The main cause of revision is for aseptic loosening. Another reason for revision is dislocation of the components. Dislocation continues to be a problem in total hip replacement. Every surgeon who performs this operation has this complication, no matter how experienced he or she is with total hip replacement. The incidence of dislocation varies widely in the community. Most experienced surgeons expect less than a 3% incidence. Some community surgeons are known to have as high as 30% dislocation rate. Dislocations are proven to be associated with (i) the type of approach, posterior being the most common--90%, and (ii) whether the case is a revision (three times the incidence over primary replacements). About half of the dislocations occur in the first month after the procedure. The remainder of the dislocations occur over the span of many years. A dislocation can occur at any time in the lifetime of the arthroplasty. It is evident that dislocation remains a problem complication in the most successful orthopaedic procedure ever devised.
Attempts have been made to make the hip replacement operation more stable and thus reduce the incidence of dislocation. To preserve soft tissue attachments posteriorly, the surgical approach has been changed. The "direct lateral approach" has been utilized in the joint replacement, but residual pain, limp, and heterotopic ossification have tempered its use. It is not well proven that the incidence has been dramatically affected despite this change in the approach. Recently, unpublished reports have demonstrated an improved dislocation rate through the reattachment of soft tissues in conjunction with the posterior approach. This technique remains to be proven and additional study is needed. The posterior soft-tissue techniques do not address revision operations, since the soft tissues are deficient or absent in revision cases.
Modification of the components has also been undertaken to try and make the arthroplasty more stable. The use of a locking mechanism between the femoral ball and the acetabular socket has been fraught with problems. This so-called "constrained" socket fails early because of high stresses arising from impingement between the socket and femoral neck. This may cause the socket to pull out from the bone attachment in the pelvis, or dislodge the ring lock holding the devices together. Another modification is the simple addition of a buildup of the polyethylene rim. This modification does not appear to influence the dislocation rate in clinical studies. This is probably because of the reduction of a "safe zone" before impingement occurs between the femoral neck and the rim of the polyethylene socket. On the femoral side, attempts at enlarging the diameter of the femoral head had not influenced the rate of dislocation. Enlarging the femoral head has been unpopular largely because of the problem of increased polyethylene wear seen with large diameter heads.